1. Field of the invention
The present invention is generally concerned with digital systems for transmitting speech signals, in particular mobile radio systems, in which a bit rate reducing code is used by the transmitter and a corresponding decoder is used by the receiver.
The present invention is more particularly concerned with systems of this kind in which said bit rate reducing code is a so-called block code which, in the manner that is known in itself, extracts representative parameters of speech signals in time intervals of specified duration (typically 20 ms) during which the characteristics of the speech signals can be regarded as stable, and transmits these parameters, rather than the signals themselves, in the form of entities called blocks or speech frames.
The present invention is even more particularly concerned with systems of this kind in which the receiver estimates the acceptable or unacceptable nature of the blocks received, a received block being deemed to be unacceptable if its transmission quality (as determined at the receiver by methods known in themselves) is deemed to be insufficient, for example.
2. Description of the prior art
Document WO 94/29849 describes replacing blocks deemed to be unacceptable with blocks obtained by interpolation of these blocks with previous blocks deemed to be acceptable and simultaneously reducing the volume, the volume being reduced to zero after a certain time if the blocks continue to be unacceptable.
A particular aim of the present invention is to provide a volume control device for a receiver of block coded speech signals adapted to receive signals transmitted using the discontinuous transmission (DTX) technique.
A particular aim gf the present invention is to provide a volume control device for a receiver of block coded speech signals that can function equally well whether or not the discontinuous transmission technique is used to transmit to the receiver.
The discontinuous transmission technique is intended in particular to reduce the level of interference in a cellular system such as the GSM system, for example. In the absence of voice activity, no information is transmitted (this is equivalent to the transmission of "empty" blocks in the description that follows) apart from "comfort" noise intended to prevent the receiver deciding that communication has been cut off. The bit rate of this noise is very much lower than that of speech information.
In a mobile radio system such as the GSM system, for example, the blocks corresponding to the comfort noise are transmitted in particular frames called silence description (SID) frames within a multiframe structure, the other frames of the multiframe being "empty" blocks (the terms "frame" and "multiframe" refer here to a time-division multiplex structure used for transmission in a system of this kind).
For more information on the discontinuous transmission technique reference may be made to "The GSM System for Mobile Communications" by Michel MOULY and Marie-Bernadette PAUTET (ISBN 2-9507190-0-7).
The "empty" blocks corresponding to frames other than the SID frames are deemed to be unacceptable blocks and are replaced, by extrapolation, with the comfort noise conveyed in the preceding SID frames. However, a problem arises with a receiver of the above-mentioned type, i e. a receiver in which the volume is reduced, possibly to zero, in the event of unacceptable blocks, namely that listener comfort is not optimized because the reproduction of the comfort noise is not optimized.
The present invention avoids this drawback.